This invention relates to a catalytic cracking process and is particularly concerned with converting high molecular weight hydrocarbon feedstocks that contain substantial quantities of vanadium and other metal contaminants to lower molecular weight products utilizing separate particles of a metals scavenging composition in combination with the cracking catalyst.
Fluidized catalytic cracking (FCC) units are used in the petroleum industry to convert high boiling hydrocarbon feedstocks to more valuable hydrocarbon products, such as gasoline, having a lower average molecular weight and a lower average boiling point than the feedstocks from which they were derived. The conversion is normally accomplished by contacting the hydrocarbon feedstock with a moving bed of catalyst particles at temperatures ranging between about 800.degree. F. and about 1100.degree. F. The most typical hydrocarbon feedstock treated in FCC units comprises a heavy gas oil, but on occasions such feedstocks as light gas oils, naphthas, reduced crudes and even whole crudes are subjected to catalytic cracking to yield lower boiling hydrocarbon products.
Catalytic cracking in FCC units is generally accomplished by a cyclic process involving separate zones for catalytic reaction, steam stripping, and catalyst regeneration. The hydrocarbon feedstock is blended with an appropriate amount of catalyst particles to form a mixture that is then passed to a catalytic reactor, normally referred to as a riser, wherein the mixture is subjected to a temperature between about 800.degree. F. and about 1100.degree. F., normally between about 900.degree. F. and 1050.degree. F., in order to convert the feedstock into gaseous, lower boiling hydrocarbons. After these lower boiling hydrocarbons are separated from the catalyst in a suitable separator, such as a cyclone separator, the catalyst, now deactivated by coke deposited upon its surfaces, is passed to a stripper. Here the deactivated catalyst is contacted with steam to remove entrained hydrocarbons that are then combined with the vapors exiting the cyclone separator to form a mixture that is subsequently passed downstream to other facilities for further treatment. The coke-containing catalyst particles recovered from the stripper are introduced into a regenerator, normally a fluidized bed regenerator, where the catalyst is reactivated by combusting the coke in the presence of an oxygen-containing gas, such as air, at a temperature which normally ranges between about 1000.degree. F. and 1500.degree. F. The cyclic process is then completed by blending the reactivated catalyst particles with the feedstock entering the riser of the FCC unit.
There has recently been a strong trend in the petroleum industry toward the use of higher molecular weight hydrocarbons, e.g., residual oils, as feeds to FCC units. A common problem arising from the use of such feedstocks is that they normally contain relatively high concentrations of metals, such as nickel, vanadium, iron and copper, which tend to deposit on the catalyst during cracking operations, thereby gradually reducing its activity and selectivity. These metals are normally present in the form of organometallic compounds, such as porphyrins and asphaltenes (where the metals are coordinated to heteroatom elements). Some of the metal contaminants which deposit on the catalyst, such as nickel constituents, deleteriously affect selectivity by increasing the production of hydrogen, light gases and coke. Other metal contaminants, such as vanadium, decrease activity by destroying the zeolite portion of the catalyst. According to U.S. Pat. Nos. 4,432,890 and 4,485,184, the disclosures of which are hereby incorporated by reference in their entireties, vanadium is especially detrimental to catalyst life. These patents teach that, as the vanadium oxide level on the catalyst increases, the high temperatures encountered in the catalyst regenerator cause vanadium pentoxide (V.sub.2 O.sub.5) to melt and begin to flow. The resultant liquid vanadium pentoxide enters the zeolite structure, irreversibly destroying the structure of the zeolite so as to form a less active amorphous material. The melting and the flowing of the vanadium pentoxide can, in some cases, also cause coalescence among catalyst particles which adversely affects fluidization properties.
Accordingly, it is an object of the present invention to provide a fluid catalytic cracking process for treating feedstocks that contain relatively high concentrations of vanadium and other metals while maintaining the activity and selectivity of the catalyst at useful levels. It is another object of the invention to provide a metals scavenging composition for use in such a fluid catalytic cracking process. These and other objects of the invention will become more apparent in view of the following description of the invention.